1. Field of the Invention
This invention relates to the field of separation processes. More particularly, it pertains to reverse osmosis processes and to a process for producing a unique reverse osmosis membrane that exhibits a significantly high tolerance to chlorine and displays resistance to
2. Description of the Prior Art
Reverse osmosis (RO) has been considered a viable method of obtaining commercial quantities of potable water for quite some time. The process is relatively straight forward. A volume of impure water, such as brackish water or sea water, is introduced against a semipermeable membrane and pressure is applied to the water to overcome or reverse the osmotic pressure thereacross. High fluxes (through puts) can be achieved without phase change and under isothermal conditions so that no heating or cooling is required.
Although there still is differing opinion on the method of semi-permeability in RO membranes, the majority of models follow the solubility-diffusion-imperfection model proposed by Applegate and Antonson.sup.1 in 1972. The model assumes that the water and ions are transported through the semi-permeable membrane via a solubility-diffusion mechanism. An imperfection term is allowed that accounts for flaws that permit undiluted salts and particulate to pass the barrier. The transport equations are: EQU J.sub.v =k.sub.1 (.DELTA.P-.DELTA..pi.)+k.sub.3 .DELTA.P EQU J.sub.s =k.sub.12 .DELTA..pi.+k.sub.3 .DELTA.PC.sub.0
where J.sub.v is the volume flux; J, is the salt flux; k.sub.1, k.sub.2 and k.sub.3 are transport coefficients; P is the pressure difference across the membrane; EQU .DELTA..pi.=.pi..sub.brine -.pi..sub.permeate
and C.sub.0 is the salt concentration.
The salt rejection then can be shown as: EQU R=[C.sub.0 .DELTA.P(1+.alpha.)+.pi..sub.0 (C.sub.0 +.beta.)-[C.sub.0.sup.2 .DELTA.P.sup.2 (1+.alpha.)+2C.sub.0 .DELTA.P.pi..sub.0 EQU (1+.alpha.)(C.sub.0 +.beta.)+.pi..sub.0.sup.2 (C.sub.0 +.beta.).sup.2 -4.pi..sub.0 .DELTA.PC.sub.0.sup.2 ]1/2/2.pi..sub.0 C.sub.0
where .alpha.=k.sub.3 /k.sub.1,.beta.=k.sub.2 /k.sub.1 and .pi..sub.0 is the osmotic pressure on the brine side of the membrane. Previous researchers.sup.2 have shown that the solvent transport coefficient, k.sub.1, is inversely proportional to the log of an activation energy for transporting the solvent through the membrane. Experimental data.sup.3 has shown that as salt concentration increases, the value of k.sub.1 always decreases. It is thought.sup.4 that the semi-permeable membrane undergoes a hydration-dehydration process effecting this transport coefficient. FNT .sup.1. T. Wydeven, A Survey of Some Regenerative Physico-Chemical Life Support Technology (NASA), NASA Technical Memorandum 101004, November 1988. FNT .sup.2. G. P. Muldowney and V. L. Punzt, A Comparison of Solute Rejection Models in Reverse Osmosis Membranes for the System Water-Sodium Chloride-Cellulose Acetate, Ind. Eng. Chem. Res. 1988, 27, 2341-2352. 1988. FNT .sup.3. D. L. Kronmiller, unpublished research. FNT .sup.4. S. G. II'Yasov, I. N. Kalvina, G. A. Kyulyan, V. F. Moskalenko, and E. P. Ostapchenko, Sov. J. Quantum Electron ., 4, 1287 (1975).
The terms k.sub.2 and k.sub.3 changes are more complex, but they seem to collectively decrease to produce a decrease in salt flux approximately equal to the volume flux decrease. Therefore, for isobaric operation, rejection is independent of salt concentration.
Table 1 shows typical transport coefficients for cellulose acetate and polyamide membranes.
TABLE 1 ______________________________________ TRANSPORT COEFFICIENTS FOR CELLULOSE ACETATE AND POLYAMIDE MEMBRANES Membrane k.sub.1.sup.a .times. 10.sup.10 k.sub.2.sup.b .times. 10.sup.12 k.sub.3.sup.a .times. 10.sup.12 ______________________________________ Cellulose acetate 0.26 0.13 0.03 TF Polyamide 3.45 1.18 1.53 ______________________________________ .sup.a is in units of liters/cm.sup.2sec-psi. .sup.b is in units of moles NaCl/cm.sup.2sec-psi.
The most widely used form for the production of high purity water is the spiral wound membrane (see FIG. 1 ). The spiral wound construction results in high surface area, which yields substantially higher flux per membrane unit. Further, the physical spacing between semi-permeable leaves, that make up the spiral wound form, provide a flow characteristic that diminishes the physical fouling problem.
Reverse osmosis membranes have become the liquid permeation separation process of choice over the last two decades. They have taken on several different physical forms as dictated by the application process. They currently are found in tubular form used widely for waste management and food processing. Hollow fiber membranes have been used in limited applications but suffer inherently from the fouling resulting in a substantial cost disadvantage.
Dual layer (DL) membranes have had limited application. The dual layer membrane is made from a mixture of zirconium oxide and polyacrylic acid deposited on the interior of a porous metal or ceramic tube.sup.5. DL membranes have a high fouling rate and are not chemically stable under many operating conditions. FNT .sup.5. T. Wydeven, A Survey of Some Regenerative Physico-Chemical Life Support Technology (NASA), NASA Technical Memorandum 101004, November 1988.
The first significant commercial applications for RO membrane use were brackish water purification and sea water desalination. The first semi-permeable membranes used were of cellulose acetate construction. These membranes were limited in use to waters having a pH of 4 to 7. Further, high feed pressures were required to obtain significant flows. The high pressure was not only costly but also limited the life due to physical deformation of the semi-permeable membrane.
In 1971, Richter and Hoehn.sup.6 patented a permselective, aromatic, nitrogen-containing polymeric membrane which initiated a new class of practical reverse osmotic membranes which require lower feed pressures and produce higher purity water. Salt rejection of greater than 99% was finally available. Further, the chemical composition resulted in a membrane which was more stable over wide ranges of pH. However, the new semi-permeable membrane was intolerant of halogens such as chlorine, which is routinely used in biostatic treatment of water. FNT .sup.6. J. W. Richter and It. H. Hoehn, U.S. Pat. No. 3,567,632 (1971).
Further, pressures of 6875 kPa (1000 psi) were still required.
The introduction by Filmtec (now owned by DOW, Inc.) of the FT30 thin film polyamide semi-permeable membrane brought about the most recent economical changes in RO membrane characteristics. Pressures of only 1550 kPa (225 psi) were necessary to produce water of greater than 99% salt rejection at acceptable flow rates for industrial application. As industry applied these RO membrane systems to various high purity water needs, many process problems were solved. However, the membranes still have chlorine tolerance problems and fouling problems.sup.7. FNT .sup.7. D. L. Kronmiller, Membrane Scaling, Controlling Silicates in RO Water Systems, Ultrapure Water, March 1992.
To increase the efficiency and economical potential for reverse osmosis high purity water systems, a new semi-permeable membrane material must be sought that has the necessary physical characteristics. Polymers based on polyvinylpyrrolidone have shown promise in ultrafiltration and microfiltration applications.sup.8 by cross-linking the hydrophilic polymer after fixation on an appropriate substrate of flat or tubular hollow fiber structure. Once again this hollow fiber structure is limited by the fact that it is easily fouled. The semi-permeable membrane obtained has high thermal stability, improved chemical resistance, good mechanical durability. However, the composition of the polymer resulted in pore sizes too large to produce the high water purity. FNT .sup.8. D. W. Hendrick, A. Cornelis, Smolders, U.S. Pat. No. 4,798,847 (1989).
The several advantages of laser-initiated polymerization are important to the development of a viable semi-permeable membrane. Selective bond activation prevents dispersing the energy to all degrees of freedom leading to random thermal heating which can cause thermal degradation; eliminates unwanted competing side reactions which lead to impurities that diminish the semi-permeable membrane characteristics.